The present invention relates to a delay line frequency discirminator, more particularly used for stabilizing an oscillator in a microwave equipment.
The low power oscillator used in the microwave equipments and requiring a good frequency stability are often of the quartz type. They then have at least one stage supplying a signal at a frequency corresponding to the selected crystal followed by a frequency multiplier making it possible to obtain frequencies of approximately 10 GHz. The output of this frequency multiplier is often connected to an amplifier making it possible to obtain the desired power. However, these oscillators have two disadvantages which can cause problems in certain equipment. On the one hand the frequency is fixed and on the other hand the phase noise inherent in the basic quartz oscillator rises in proportion to the frequency multiplication, which is disturbing in the case of very high frequencies.
It is also possible to use resonant circuit oscillators comparable to those used in radio equipment operating at medium and high frequencies. In the microwave frequency range they can include cavities or line resonators on a substrate using the so-called microstrip or "stripline" technology, with active elements such as Gunn or avalanche diodes. However, these oscillators do not have an adequate phase and frequency stability for certain equipment.
According to the prior art, this frequency stability of line oscillators and active elements of for example the Gunn or avalanche diode type is obtained by means of a frequency locking loop using an acoustic delay line. In this way, so-called noise compression oscillators are obtained.
These acoustic lines are expensive and are at present limited to frequencies in the X band in which quartz oscillators have better noise, frequency and phase performances. Moreover, their power is greatly limited and their losses can rise to 30 or 40 dB, whilst also causing a significant delay .tau. creating ambiguities in the reference frequency equal to 1/.tau.. This ambiguity can be every 2 MHz if .tau.=0.5 .mu.sec.